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I'm not sure of it's accuracy, but there's a calculator at energysavingtrust.org, and here's a very similar calculator that's a little bit easier to read (imo). I'm guessing the location factor for both of them is based on average winter low temperatures for the region selected. I'd look up climate info on the areas and try to pick something similar to where you live.

Thanks, I tried these calculators, they are not bad, however they don't take into account how fast you wish a home to worm up after you have been out at work and they seem to think the -3 is as code the the England get. (It was until last year and the last few weeks of this year...)
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WalkerDec 17 '10 at 9:18